1. Field of the Invention
This invention relates to a pixel driving circuit for a light emitting display, and more particularly to a pixel driving circuit which is preferably used in a light-emitting display such as an organic EL (electro-luminescence) display which uses a current driving type transistor for driving a pixel.
2. Description of the Related Art
The light-emitting display such as an organic EL display requires a current driving transistor (TFT) to drive an element arranged for each of pixels. As a generally driving technique, the gate potential of the current driving transistor is controlled to follow a video signal so that the gradation of an image is controlled.
However, large characteristic variations among the transistors for driving an organic EL element lead to variations in a driving current among pixels, which seriously affects the display quality.
The characteristic variations among the driving transistors resultantly lead to the variations in the driving current among the pixels. This deteriorates the quality of a displayed image so that the image appears as if noises were roughly scattered on the image.
On the other hand, in case of adopting a circuit configuration in which a model current is injected form the source side of the driving transistor, while a selected pixel is subjected to current programming, it is necessary to stop current supply to other pixels.
Therefore, since any pixel cannot be set in a driving mode until the current programming operation for all the pixels is completed, it is necessary to wait until the current programming operation for all the pixels is completed. Namely, the period until the current programming operation for all the pixels is a waiting time which does not permits the EL to be driven in timing.
Further, in case that a programmed current value is constant and the light emitting element is turned on/off to exhibit the gradation, it is necessary that one frame is divided into some sub-frames to control the sub-frames at high speed. In the technique according to the related art, the useless waiting time is required, the sub-frame time is correspondingly lengthened and the upper limit of the addressing speed of the entire screen is lowered. Accordingly, the number of the sub-frames contained in one frame is decreased so that the number of exhibitable gradations is lowered to deteriorate the display image quality.
This invention has been accomplished in view of the above circumstance. This invention intends to provide a pixel driving circuit for a light emitting display in which during current programming, the drain side of a driving transistor is once separated from the light emitting element to be driven, a model current is injected into the drain side so as to program, and the drain side is connected to the side of the light emitting element after the current programming has been completed so that the pertinent pixel can be set in a driving mode, thereby removing a useless waiting time and simplifying the entire driving operation.
In order to solve the above problem, according to a first aspect of the invention, there is provided a pixel driving apparatus for a light emitting display comprising:
a light emitting element;
a driving transistor having a drain, a source, and a gate;
a current programming means for injecting a model current into one of the drain and the source of the driving transistor to program a gate voltage generated in response to the injection of the model current therein; and
a light emitting element driving means for connecting the one of the drain and the source of the driving transistor to a side of the light emitting element to be driven after programming of the model current by the current programming means has been completed to drive the light emitting element.
According to the above configuration, the operating of programming the model current causes the pixel to program the model current as a current value so that the driving current can be controlled intentionally regardless of a variation in the characteristic of the driving transistor. Accordingly, variations in the driving current among the pixels, which are ascribable to variations in the transistor characteristic peculiar to the TFT process, can be suppressed so that the display quality can be improved.
Further, while a pixel on a selected line is current-programmed, another pixel on a non-selected line can be set in a driving mode. Therefore, the useless waiting time can be removed to provide a pixel driving circuit for a light emitting display with a simplified entire operation.
According to a second aspect of the invention, there is provided the pixel driving apparatus according to the first aspect of the invention, wherein the current programming means comprises:
a model current source for injecting the model current into the one of the drain and the source of the driving transistor;
a first switching means connected between the light emitting element and the one of the drain and the source of the driving transistor;
a first means for injecting the model current from the model current source into the one of the drain and source of the driving transistor through the first switching means in a state where the light emitting element is separated from the circuit; and
a voltage accumulating means for accumulating the gate voltage.
According to a third aspect of the invention, there is provided the pixel driving apparatus according to the second aspect of the invention, wherein the light emitting element driving means comprises:
a second and a third switching means which are connected between the model current source and the one of the drain and the source of the driving transistor and connected between the model current source and a gate of the driving transistor, respectively;
a current supplying means for separating the model current source from the one of the drain and the source of the driving transistor through the second switching means to supply a current to the emitting element by the gate voltage accumulated in the voltage accumulating means; and
a second means for holding supply of the gate voltage by the voltage accumulating means through the third switching means until an operation of the current programming means becomes valid.
According to a fourth aspect of the invention, there is provided the pixel driving apparatus according to the first aspect of the invention, wherein the current programming means comprises:
a connecting means for setting the potential of the other of the drain and the source line of the driving transistor in a state where the light emitting element cannot operate to connect the model current source to the one of the drain and the source of the driving transistor;
a first means for injecting the model current from the model current source into the one of the drain and the source of the driving transistor through the connecting means in a state where the light emitting element is separated from the circuit; and
a voltage accumulating means for accumulating a gate voltage generated in response to the injection of the model current.
According to a fifth aspect of the invention, there is provided the pixel driving apparatus according to the fourth aspect of the invention, wherein the light emitting element driving means comprises:
a second and a third switching means which are connected between the model current source and the one of the drain and the source of the driving transistor and connected between the model current source and a gate of the driving transistor, respectively;
a current supplying means for separating the model current source from the one of the drain and the source of the driving transistor through the second switching means and setting the potential of the other of the drain and the source line in a state where the light emitting element can operate to supplying a current to the light emitting element by the gate voltage accumulated in the voltage accumulating means; and
a second means for holding supply of the gate voltage by the voltage accumulating means through the third switching means until an operation of the current programming means becomes valid.
According to a sixth aspect of the invention, there is provided a pixel driving apparatus for a light emitting display comprising:
a light emitting element;
a driving transistor;
a model current source adapted to supply a model current;
a first switching section adapted to connect/disconnect the one of a drain and a source of the driving transistor and the light emitting element;
a second switching section disposed between the model current source and the one of the drain and the source and adapted to connect/disconnect the model current source and the one of the drain and the source;
a third switching section disposed between the model current source and a gate of the driving transistor and adapted to connect/disconnect the model current source and the gate; and
a voltage accumulating section disposed between the third switching section and the gate and adapted to accumulate a voltage between both ends of the driving transistor.
According to a seventh aspect of the invention, there is provided the pixel driving apparatus according to the sixth aspect of the invention, wherein the first switching section is a switching element disposed between the one of the drain and the source and the light emitting element.
According to an eighth aspect of the invention, there is provided the pixel driving apparatus according to the sixth aspect of the invention,
wherein the first switching section is a power source connected to the other of the drain and the source of the driving transistor;
the power source applies a low voltage to the other of the drain and the source to disconnect the drain and the light emitting element, the low voltage not enabling to drive the light emitting element; and
the power source applies a high voltage to the other of the drain and the source to connect the drain and the light emitting element, the high voltage enabling to drive the light emitting element.
According to a ninth aspect of the invention, there is provided the pixel driving apparatus according to the seventh aspect of the invention, wherein the first and second switching sections and the switching element are transistors.